This invention relates to droplet aerosol collection by electrostatic precipitation, and methods that improve efficiency for particle collection. The improvements include one or more of the use of multiple, thin wire discharge electrodes; the use of a conductive porous medium as a collecting surface; the use of high voltage electrostatic shield to prevent particle deposition on the insulator for the components; and the use of heated insulator to prevent vapor condensation and particle deposition by thermophoresis.
Electrostatic precipitation is one of the most widely used methods for removing suspended particulate matter from a gas for gas cleaning or air pollution control. In comparison with other particulate collecting devices, such as cyclones, wet scrubbers, filters, and the like, an electrostatic precipitator has the advantage of low pressure drop, high collection efficiency and requiring relatively small amounts of electrical power for its operation. The low pressure drop of the electrostatic precipitator makes the device most advantageous with large volumetric flow rates of the gas flow needing treatment. Electrostatic precipitation have been used extensively for large scale industrial applications, such as removing fly ash from power plants, controlling particulate emission from smelters, steel and cement making and other similar industries, and general purpose air cleaning for building ventilation. A typical electrostatic precipitator may operate at several hundred cubic feet per minute of flow in small systems, to several million cubic feet per minute for large industrial installations.
The first laboratory demonstration of electrostatic precipitation was made by Hohifeld in 1824, according to credible sources. The first U.S. patent on electrostatic precipitation was issued to Walker in 1886 as No. 342,548. Numerous other electrostatic precipitator patents have been issued over the years. Those considered the most significant include U.S. Pat. No. 895,729 to Cottrell on the use of rectified alternating current for electrostatic precipitation, and the invention of the liquid film precipitator by Bums as shown in U.S. Pat. No. 1,298,088; the fine wire electrode and two-stage precipitation system of Schmidt, U.S. Pat. No. 1,329,285; the low-ozone air-cleaning precipitator of Penney U.S. Pat. No. 2,000,654; and pulse energizing of precipitators disclosed in U.S. Pat. No. 2,509,548 to White, among others.
The fundamental design of the electrostatic precipitator has remained relatively unchanged over the years. In its simplest form for a single stage precipitator, a high voltage electrode is placed in the center of a grounded tube. A high DC voltage on the small diameter center electrode causes a corona discharge to develop between the electrode and the interior surface of the tube. As the gas containing suspended particles flows between the electrode and the wall of the tube, the particles are electrically charged by the corona ions. The charged particles are then precipitated electrostatically by the electric field onto the interior surface of the collecting tube.
One disadvantage of the electrostatic precipitator is its relatively large physical size. According to Deutsch (W. Deutsch, Ann. der Physik, Volume 68, p. 335, 1922), the basic equation governing the operation of the electrostatic precipitator is: EQU .eta.=1-e.sup.-A w/Q
The Deutsch equation relates the precipitator collection efficiency, .eta., to the collecting area of the precipitator, A, the volumetric flow rate, Q, through the precipitator, and the electrical migration velocity, w, of the particles. e is the constant, 2.718, the base of natural logarithms. For a specific application, the collecting area of the precipitator, A, is determined when the required volumetric gas flow rate, Q, is known. To reduce the overall physical size of the precipitator, closely spaced precipitating plates can be used. However, there is a limit on this approach to reducing physical size. When the resulting physical size of the precipitator is still too large for the application, an electrostatic precipitator is then considered unfeasible.
Several applications have developed in recent years where a significant reduction in the overall physical size of the electrostatic precipitator is needed. One application is the removal of the suspended particulate matter from the blowby gas from a Diesel engine. In Diesel engines, the high temperature, high pressure combustion gas in the engine cylinder has a tendency to leak past the piston rings into the crankcase. This is usually referred to as the blowby gas. This blowby gas contains lubricating oil droplets from the lubricating oil films atomized by the high velocity blowby gas flowing from the high pressure cylinder into the crankcase. It also contains Diesel exhaust particulates, which result from the incomplete combustion of the Diesel fuel in the engine cylinder. The amount of blowby gas is relatively small for new engines, but will increase over time as the engines age, and the piston rings no longer provides a good seal. This blowby gas usually has a flow rate of few cubic feet per minute to perhaps as high as 20 cfm for engines in good operating condition.
The Diesel blowby gas is currently being exhausted directly into the atmosphere. In order to protect the environment, there is a need to remove suspended oil droplets and Diesel exhaust particulates in the blowby gas so that the blowby gas can be returned to the fresh air intake side of the Diesel engine for further combustion. This "blowby gas recirculation system" is practical only when the suspended particulate matter is removed to avoid contaminating the components and equipment located on the air intake side of the Diesel engine. One such component is the turbo charger or compressor used to supercharge the Diesel engine to increase its power output and efficiency.
For application in the blowby gas recirculation system, the electrostatic precipitator must be compact and reliable. It is also desirable that the operating voltage of the precipitator be relatively low so that very a high supply voltage is not needed.
Another application for an electrostatic precipitator that is reduced in size from existing precipitators is for removing suspended oil and grease particles in the exhaust gas from commercial kitchens, including kitchens in fast-food, as well as conventional, restaurants.
A third application of an electrostatic precipitator of reduced size is to remove cutting fluid droplets from the machine shop environment. During machining of metal parts, a cutting fluid is usually directed at the tool and the parts being machined to provide cooling as well as lubrication. Some of this cutting fluid is aerosolized to form small droplets by the higher speed rotary cutting tool. This cutting fluid aerosol presents a heath hazard to the workers and must be filtered to remove the suspended droplets. Conventional fibrous filters are not suitable for this application, because the collected droplets tend to clog the filter and produce excessive pressure drop in a short time. The inherent advantage of the small compact physical size and the inherent flame arresting properties of the precipitator of the present invention makes it particularly suited for these applications.
It should be noted the term "compact size" is used here in a relative sense to indicate that the size of the precipitator designed on the basis of this invention is smaller or more compact in comparison with electrostatic precipitators of a conventional design at the same flow rate and at the same efficiency level. By necessity, as a diesel blowby particle collector, the electrostatic precipitator must be sufficiently small to fit under the hood of a truck powered by a diesel engine. The overall volume of the collector must be no more than a few liters, preferably below two liters. On the other hand, an electrostatic precipitator designed for kitchen exhaust applications will need to be considerably larger because of the high flow rate of the exhaust gas to be treated. Such a collector can also be called compact even though the collector is several cubic feet in total volume so long as the collector of the conventional design is even larger, perhaps by as much as 50 or 100%.